Product modelling is an essential part of all computerised design and
manufacturing activities. A precise mathematical model of the geometry of
products is important, but must be supplemented with technological information
such as the material, mechanical properties, functional specifications and
tolerances. Modern CAD systems can model and manipulate components with
complex geometry. However, technological information is represented as text
symbols on the computer screen or drawing, and subsequent application
programs are frequently unable to use this information effectively. This paper
discusses this problem, and establishes the geometric elements required for the
representation or dimensions and tolerances in a feature-based product
modelling environment